Unknown men riding bikes Sunday hurled a hand grenade at a businessman’s house in a Karachi’s posh area. Reportedly, the grenade turned out to be a dud and did not go off. According to sources, extortionists were behind the attack as they have been threatening the businessman, a resident of Khayban-e-Mujahid Defence Housing Authority (DHA) with dire consequences for his not submitting to their demands. Police have started investigation.